Transmitting real-time video data over a wireless network is a challenge because wireless networks typically have lower bandwidth and experience high bit error rates due to factors such as long and short fades, shadowing and environmental noise, none of which are conducive to the transmission of time sensitive data, such as real-time video. A common approach to compensate for these factors is to compress the video data, thus reducing the amount of data being transmitted. However, existing compression schemes, such as those defined by the Motion Picture Expert Group (MPEG), tend to aggravate the errors caused by transmission by propagating the errors throughout the video data.
Typical video compression schemes generally involve storing only changes from one video frame to another. These are commonly referred to as predictive video coding schemes, wherein the coding of one frame depends upon information from another. These schemes tend to introduce further data loss at least in part because if errors are introduced into the coding of the first frame, the error propagates to others due to the interdependency between one frame and another. This is generally referred to as “temporal error propagation.” This may cause noticeably low quality video frames.
One approach known in the art to stop the propagation of errors is to refresh or intra-code frames, which are referred to as intra-frames (“I-frames”) within the video data. An I-frame is a single frame of digital content that is encoded independent of the frames that precede and follow it, i.e., all of the data needed to display that frame by itself is stored. These frames are typically interspersed with inter-frames, also referred to as predictive frames (P-frames), which are the frames that include changes compared to the previous frames and are interdependent between one another. Each I-frame is usually followed by a sequence of P-frames, and the group of frames are collectively known as a group of picture (“GOP”). One GOP is an independent decodable entity.
An advantage of using I-frames is that the propagation of errors terminates at the beginning of each GOP with the I-frame. However, for low-bandwidth (especially wireless) video transmission (e.g. 128 kbit/s) environments, the use of I-frames creates obstacles. Because I-frames are self-contained, I-frames require a larger number of bits for representation, compared to P-frames. This, in turn, may cause a higher delay in transmission. To compensate for this delay, subsequent P-frames are skipped, which may cause “motion jerkiness” in the video. Therefore, in most encoders designed for wireless use, only one I-frame is used at the beginning of the sequence, followed by all P-frames. However, because only one I-frame is used, errors introduced into the data during encoding may propagate throughout the encoding of the P-frames, jeopardizing the fidelity of the decoded video. Thus, some form of data refreshing, i.e., intra-coding, may be required to take place continually to halt the propagation of errors, particularly temporal errors.
Over the past few years, researchers have proposed a variety of approaches to increase the robustness of low bit-rate video communications, such as those described in “Error control and concealment for video communication: A review” by Y. Wang and Q.-F. Zhu, Proc. IEEE, vol. 86, pp. 974-997, May 1998 and “Resynchronization of Motion Compensated Video Affected by ATM Cell Loss” by P. Haskell and D. Messerschmitt, Proc. International Conference of Acoustic, Speech, and Signal Processing, San Francisco, pp. 545-548, March 1992. One approach involves selectively refreshing, or intra-coding, a number of macroblocks (“MBs”), which are essentially blocks of a frame, which can have any size but are often 16×16 pixels in size. Intra-MBs, similar to I-frames, are self-contained blocks of data that do not depend upon information from MBs in other frames. Subsequent MBs may then be inter-MBs, which include only changes from the corresponding MBs in the previous frame video frame. This approach avoids the use and the disadvantages of I-frames; however, some techniques randomly select MBs to intra-code, while others indiscriminately intra-code MBs without any type of error monitoring system. Thus, these techniques tend to still allow the propagation of errors throughout the frames. Accordingly, an improved video compression system is desirable.